Osteosarcoma is the most common bone tumour found in the dog. Osteosarcoma primarily affects middle-aged large and giant purebred dogs. The disease is extremely aggressive and has a tendency to spread rapidly into other parts of the dog’s body.
Causes of Osteosarcoma
To date, little is known about the aetiology of osteosarcoma. Rapid skeletal growth and large body size have been hypothesized to be relevant, given that most osteosarcomas occur during the pubertal years. It is speculated that sex hormones may be important, although it is unclear how these hormones may play a role in the development of osteosarcoma. Males are more often affected and may have a poorer prognosis. Similar to the situation in humans, relatively little is known about factors that influence the development of osteosarcoma in dogs. Years ago, a study showed that the risk of bone sarcoma in large and giant breed dogs exceeds that of small breed dogs by as much as 185-fold. Breeds with an apparent increased risk of developing osteosarcoma include the Rottweiler. Osteosarcoma most likely develops from a number of factors including genetic predisposition and environmental factors such as radiation, microtrauma, and nutrition.
Symptoms of Osteosarcoma
The bones of the limbs are affected much more often than bones of the spine, skull, or ribs. Osteosarcoma of the limbs is called appendicular osteosarcoma. The most common sites occurring away from the elbow and close to the knee. Dogs with osteosarcoma show various degrees of pain and lameness which can be intermittent or constant. Obvious swelling of the bone may become evidence as the tumour progresses and grows beyond the normal bone. Because tumourous bone is not as strong as normal bone, some dogs may fracture the bone with only minor injury.
Initial evaluation of a dog with a suspected bone tumour includes a physical examination, bloodwork, and radiographs (X-rays) of the bone and lungs. Radiographs generally show a classic pattern of bone loss (lysis) and bone production (proliferation). Because this radiographic appearance can also be caused by other types of bone cancer or non-cancerous diseases, a biopsy of the bone is required to make a diagnosis of osteosarcoma. Biopsy entails the surgical collection of a small core of tissue from the abnormal bone. Sometimes a bone tumour is surrounded by an area of inflammation and it may be difficult to obtain a diagnosis and several samples must be taken.
Evaluation needed for a dog with Osteosarcoma
Once a diagnosis of osteosarcoma is made, it is important to determine the extent of the tumour, i.e. to perform tumour staging. Staging is performed in order to determine the best way to treat the cancer and to provide information regarding prognosis. Staging entails a thorough radiographic evaluation of the lungs, evaluation of the skeleton for other potential affected bones, and other radiographs and blood tests to help determine the best treatment protocol. Based on the results of this clinical staging, dogs with osteosarcoma can be subdivided into three categories: (1) dogs with disease confined to the limb, which accounts for the majority of cases; (2) dogs with lung metastasis at diagnosis; and (3) dogs with multiple bone lesions at presentation. Treatment is based upon the results of clinical staging at the time of diagnosis.
The treatment of osteosarcoma involves two aspects: treating the pain caused by the bone tumour and fighting the spread of the cancer (metastasis). Treatment of the primary tumour usually involves amputation of the affected limb, but limb-sparing procedures may be an option in selected cases. Amputation or limb-sparing surgery removes the primary tumour which serves two purposes: removal of a source of pain and a source of tumour cells which are continually being shed to the rest of the body. The removal of the painful limb often dramatically improves quality of life. After amputation, recovery from surgery and adaptation to life on three legs is generally fast. However, for dogs in which surgery is not an option due to other orthopaedic conditions, radiation therapy of the affected limb can be used to control pain in some patients.
How rapidly dogs with osteosarcoma succumb to metastatic disease after removal of the primary tumour is influenced by treatment with chemotherapy. Most dogs tolerate chemotherapy well, with only mild side effects such as decreased appetite and occasional vomiting and diarrhoea. Chemotherapy is not likely to cure most dogs with osteosarcoma, but can prolong survival with a good quality of life.
Prognosis for dogs with Osteosarcoma
Osteosarcoma is unfortunately a fast spreading tumour – by the time the tumour is found in the limb, it is considered to have already metastasized. Over 90% of dogs with osteosarcoma of the limb will die with metastatic disease, most often to the lungs. Dogs treated with limb amputation alone will survive approximately 4 months. With the addition of chemotherapy dogs survive 10-12 months.
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